The present invention relates generally to optical waveguides, and more particularly to methods for forming waveguides in optical substrate materials such as lithium niobate.
Techniques for fabricating optical waveguides in inorganic optical substrate materials include various methods in which a waveguide is formed by altering the index of refraction of selected portions of the substrate by ion exchange and diffusion. One well-known method of this type is the proton exchange (PE) method, used particularly on lithium niobate substrates and other crystalline materials. In. the PE method a proton-supplying exchange agent, such as benzoic acid or pyrophosphoric acid, is contacted with portions of the substrate surface, causing protons from the exchange agent to exchange with and replace some of the corresponding ions of the substrate material (e.g., lithium ions in a lithium niobate substrate) in a region near the surface of the substrate. The resulting proton-exchanged region has a higher refractive index, relative to the adjacent unaltered substrate material, for appropriately polarized light and thus can function as an optical waveguide. By selecting appropriate exchange agents and adjusting exchange conditions (and by use of a subsequent annealing step), a wide range of waveguide refractive index differences and depths can be achieved. The PE method advantageously enables relatively rapid formation of waveguides at low temperature (typically around 200xc2x0 C.) conditions, whereas other ion exchange methods, such as titanium in-diffusion, generally require much higher temperatures to achieve equivalent rates of waveguide formation. A further advantage of the PE method is that waveguides formed by PE in materials such as lithium niobate are capable of maintaining the initial polarization state of the transmitted light, whereas waveguides formed by other techniques, such as titanium indiffusion, carry orthogonal polarizations at different velocities, resulting in a change of the state of polarization of the input optical energy (e.g., from linear to elliptical).
A disadvantage associated with the PE method is that the highly acidic exchange agents conventionally employed for proton exchange may produce undesirable effects, such as surface etching of the substrate and the formation of a xe2x80x9cdead layerxe2x80x9d (a disordered, centrosymmetric region with substantially reduced nonlinear or electro-optic properties) within the resultant waveguide. These undesirable effects may be minimized or avoided by using a weakly acidic exchange agent, or by using a lithium-buffered (xe2x80x9cstarvedxe2x80x9d) exchange agent wherein a quantity of lithium ions are dispersed in the exchange agent. However, use of weakly acidic or starved exchange agents are known to reduce proton exchange rates substantially, requiring reaction times of tens or even hundreds of hours to form usable waveguides.
Roughly described, a method for forming waveguides in optical materials according to the present invention comprises the steps of providing an exchange agent including a catalyst and a proton-supplying medium, and exposing at least a portion of a surface of an optical material to the exchange agent for a specified period of time and at a specified temperature. The catalyst is selected to accelerate the rate of proton exchange in the exposed regions of the optical material, thereby shortening required exchange times and/or allowing the use of weakly acidic media in order to avoid or minimize the aforementioned problems associated with traditional highly-acidic exchange agents.
In accordance with specific aspects of the invention, the optical material comprises lithium niobate, the catalyst is selected from a group consisting of beryllium, magnesium, zinc and gadolinium ions, and the proton-supplying medium comprises an acid having a pKa greater than about 4.5 or a lithium-buffered (xe2x80x9cstarvedxe2x80x9d) acidic solution. The catalyst may be added to the proton-supplying medium by dissolving therein an ionic salt (such as a chloride) of the catalyst. The position, geometry and dimensions of resultant waveguides may be controlled by masking appropriate regions of the optical material using techniques well known in the art and by adjusting the conditions at which the proton exchange step (and any subsequent annealing steps) are performed.